CLONING AND FUNCTIONAL EXPRESSION OF A SECOND NEW AQUAPORIN ABUNDANTLY EXPRESSED IN TESTIS

A new member of water channels has been identified from rat testis. Th is gene, termed aquaporin 8 (AQP8), encoded a 263-amino-acid protein t hat contained the conserved NPA motifs of MIP family proteins. AQP8 ha s amino acid sequence identity with other aquaporins (similar to 35%) and highest with a plant water channel, AQP-gamma TIP (39%), suggestin g that AQP8 is a unique member in mammalian aquaporins. The expression of AQP8 in Xenopus oocytes stimulated the osmotic water permeability (P-f) 8.5 folds. The increase of P-f was inhibited with 0.3 mM mercury chloride by 55%, which was reversed with mercaptoethanol. The Arrheni us activation energy for the stimulated water permeability was low (5. 1 kcal/mol). AQP8 did not facilitate glycerol transport. Northern blot analysis revealed a 1.5-kb transcript of AQP8 abundantly in testis an d slightly in liver. In situ hybridization of testis revealed the expr ession of AQP8 mRNA in all stages of spermatogenesis from primary sper matocytes to spermatids in seminiferous tubules. Together with previou sly cloned AQP7, AQP8 may also play an important role in spermatogenes is. The unexpected complexity of the presence of two aquaporins in tes tis may call for the further analysis of the role of aquaporins in the reproduction biology. (C) 1997 Academic Press.